JP2002139910A - Image forming apparatus and image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To always control the toner density of a two-component developer to a prescribed set value even when environmental humidity changes by a method without using a humidity sensor. SOLUTION: The image forming apparatus consists of a photoreceptor which exposes an electrostatic charge image to light, a development part which houses the two-component developer and develops a toner image on the surface of the photoreceptor on which the electrostatic charge image is exposed, a toner replenishment part to replenish toner to the development part, an electrically conductive transferring part to transfer the toner image developed on the surface of the photoreceptor to a transfer material, a voltage feeding part to feed a prescribed bias voltage having a reverse polarity relative to the charge polarity of the toner image to the electrically conductive transfer part, a sensor for magnetic permeability to detect the magnetic permeability of the two-component developer housed in the development part, a humidity measurement part which measures the electric resistance value of the electrically conductive transfer part and measures humidity from the electric resistance value, a magnetic permeability correcting part to correct magnetic permeability changing according to humidity on the basis of measured humidity, and a toner replenishment control part to control the toner replenishment part on the basis of magnetic permeability corrected so as to make the toner density of the two-component developer always into a prescribed set value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus and an image forming method which use a copier, a facsimile, an electrostatic printer, and the like, and particularly form an image using a two-component developer.

[0002]

2. Description of the Related Art Conventionally, as a developer for forming a toner image on an electrostatic image exposed on a photoconductive photoreceptor, a toner, which is a powder ink, and a carrier which is frictionally charged with the toner are fixed. (Hereinafter referred to as "toner concentration"), and a two-component developer which does not use a carrier. In the developing method using the two-component developer, since the toner and the carrier are frictionally charged, the reverse polarity toner is less likely to be generated on the photoconductor compared to the developing method using the one-component developer. The printing quality can be obtained with less influence of the printing.

However, in the developing method using the two-component developer, when the toner concentration is reduced, the print density is reduced. When the amount of toner is reduced and the carrier is increased, the carrier is spilled from the toner replenishing section, and the toner is subsequently discharged. However, adding too much toner to the toner replenishment area will reduce the friction with the carrier, making the background fog phenomenon more likely to occur, and affecting the image quality. Need to be precisely controlled. Therefore, using a magnetic permeability sensor, the magnetic permeability of the two-component developer is detected, the toner density is calculated from the detected magnetic permeability, and when the toner density becomes equal to or less than the set value, the toner is intermittently supplied to the toner supply unit. Replenishment controls the toner concentration to an optimum level.

However, the amount of triboelectric charge between the toner and the carrier depends on the surface resistance between the toner and the carrier.
In the case of low humidity, the surface resistance increases, the amount of frictional charge increases, and the permeability increases. Conversely, in the case of high humidity, the surface resistance decreases, the amount of frictional charge decreases, and the permeability decreases. As described above, since the magnetic permeability detected by the magnetic permeability sensor fluctuates with changes in humidity, there is a problem that accurate toner concentration cannot be controlled.

[0005] As a conventional technique for solving this problem, for example, according to Japanese Patent Application Laid-Open No. 10-20710, a magnetic permeability for detecting the magnetic permeability of a developer using a humidity sensor for measuring humidity is disclosed. There has been proposed a developing device and a developing method of an electrophotographic apparatus that can stabilize print quality by controlling a toner concentration even when humidity changes by correcting a sensor humidity change.

[0006]

However, in the developing device of the electrophotographic apparatus described in Japanese Patent Application Laid-Open No. 10-20710, the cost increases because a humidity sensor for detecting humidity is separately added, and the cost of the image forming apparatus is reduced. There is a problem that the reliability of the humidity sensor is reduced due to accumulation and dew condensation of the generated ozone, silicon oil mist, toner / paper dust, and the like. In addition, when removing such dew condensation or adhered matter, for example, the removal method by heating with a heater may not be able to use such a removal method because toner fusion occurs. It becomes necessary and causes a cost increase.

The present invention has been made in view of the above circumstances. For example, by measuring humidity from the electrical resistance value of a conductive transfer portion for transferring a toner image developed on a photoreceptor to a sheet, the magnetic permeability is measured. By correcting the magnetic permeability of the two-component developer, which is detected by a sensor and changes in accordance with the humidity, the toner concentration of the two-component developer is always set to a predetermined value even when the humidity changes by a method without using a humidity sensor. Provided are an image forming apparatus and an image forming method capable of replenishing toner so as to have a value.

[0008]

SUMMARY OF THE INVENTION The present invention is directed to a developing unit for storing a photoreceptor for exposing an electrostatic image and a two-component developer for developing a toner image on the surface of the photoreceptor to which the electrostatic image has been exposed. When,
A toner replenishing unit for replenishing toner to the developing unit, a conductive transfer unit for transferring the toner image developed on the surface of the photoreceptor to a transfer material, and a conductive bias voltage having a polarity opposite to the charging polarity of the toner image A voltage supply unit for supplying the transfer unit, a magnetic permeability sensor for detecting the magnetic permeability of the two-component developer stored in the developing unit, an electric resistance value of the conductive transfer unit is measured, and a humidity is calculated from the electric resistance value. , A magnetic permeability correction unit that corrects the magnetic permeability that changes according to the humidity based on the measured humidity, and a correction so that the toner concentration of the two-component developer always becomes a predetermined set value. And a toner replenishment control unit for controlling the toner replenishment unit based on the determined magnetic permeability.

According to the present invention, the humidity is measured from the electric resistance value of the conductive transfer portion for transferring the toner image developed on the photoreceptor to the paper, detected by the magnetic permeability sensor, and changes according to the humidity. By correcting the magnetic permeability of the component developer, a method that does not use a humidity sensor can be used even if the humidity changes.
The toner can be replenished so that the toner concentration of the component developer always becomes a predetermined set value. This makes it possible to always stabilize the printing quality even when the humidity changes.

[0010] The conductive transfer section may be constituted by an ion conductive transfer roller. According to this configuration, when the ion conductive transfer roller is used, the resistance unevenness is small and the high resistance is easily formed as compared with the electronic conductive transfer roller. Further, since the resistance value changes almost linearly with the change in humidity, accurate humidity can be measured. Thereby, the magnetic permeability of the two-component developer can be accurately corrected.

The humidity measuring section supplies a reverse bias minute constant current to the conductive transfer section for a predetermined time before forming an image.
The electrical resistance value of the conductive transfer unit may be measured by measuring the reverse bias voltage applied to the conductive transfer unit. According to this configuration, a minute constant current of reverse bias is supplied during measurement of the electric resistance value, so that it can be used for a long time.
By supplying a positive bias voltage (transfer voltage) to the conductive transfer portion, it is possible to prevent internal ions from moving and prevent the electric resistance value of the ion conductive transfer roller from changing over time. Further, an effect that the toner adhered to the ion conductive transfer roller is returned to the photoconductor and cleaned is obtained, so that a special mechanism for cleaning the adhered matter is not required.

[0012] A resistance-humidity conversion storage unit in which a plurality of resistance-humidity conversion formulas for converting the electrical resistance value of the conductive transfer unit into humidity are stored in advance, and the resistance-humidity conversion storage unit stores the resistance-humidity conversion formula. A setting unit that sets a resistance-humidity conversion formula according to the humidity change characteristic, wherein the humidity measurement unit converts the electrical resistance value of the conductive transfer unit into humidity based on the set resistance-humidity conversion formula. It may be. According to this configuration, even if the electrical resistance value of the conductive transfer unit varies with respect to the same humidity due to a device variation, a resistance-humidity conversion formula is set according to the humidity change characteristic of the electrical resistance value of the conductive transfer unit. Thereby, an accurate humidity can be measured.

[0013] The apparatus further comprises a permeability / humidity change characteristic storage unit in which a humidity change characteristic of magnetic permeability with respect to a predetermined toner concentration is stored in advance, wherein the humidity measurement unit refers to the permeability / humidity change characteristic storage unit, May be configured to correct the magnetic permeability of the two-component developer detected by the magnetic permeability sensor based on the humidity measured by the method. According to this configuration, the magnetic permeability of the two-component developer can be corrected based on the accurately measured humidity even if the electric resistance value of the conductive transfer portion varies with respect to the same humidity due to a device variation. .

The magnetic permeability correction unit corrects the magnetic permeability based on the humidity measured before the image formation by the humidity measurement unit, and the toner supply control unit performs the correction during the image formation before the image formation. The toner replenishing unit may be controlled so that the toner concentration of the two-component developer always becomes a predetermined set value based on the magnetic susceptibility. According to this configuration, since the magnetic permeability is corrected based on the humidity measured before the image formation, the toner density can be controlled to a predetermined set value without lowering the printing speed.

According to another aspect of the present invention, a photoreceptor that exposes an electrostatic image and a developing unit that contains a two-component developer and develops a toner image on the surface of the photoreceptor that has been exposed to the electrostatic image When,
A toner replenishing unit for replenishing toner to the developing unit, a conductive transfer unit for transferring the toner image developed on the surface of the photoreceptor to a transfer material, and a conductive bias voltage having a polarity opposite to the charging polarity of the toner image A voltage supply unit for supplying the transfer unit, a magnetic permeability sensor for detecting the magnetic permeability of the two-component developer stored in the developing unit, an electric resistance value of the conductive transfer unit is measured, and a humidity is calculated from the electric resistance value. , A magnetic permeability correction unit that corrects the magnetic permeability that changes according to the humidity based on the measured humidity, and a correction so that the toner concentration of the two-component developer always becomes a predetermined set value. An image forming method for forming an image using an image forming apparatus including a toner replenishment control unit for controlling a toner replenishment unit based on the obtained magnetic permeability.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on an embodiment shown in the drawings. The present invention is not limited by this.

FIG. 1 is a diagram showing a configuration of an image forming apparatus according to one embodiment of the present invention. As shown in FIG. 1, the image forming apparatus according to the present embodiment includes a photoconductor 1, a charging unit 2, an exposure unit 3, a toner supply unit 4, a development unit 5, a conductive transfer unit 6, a voltage supply unit 7, a cleaning unit. 8, a static elimination unit 9, a fixing unit 10, a magnetic permeability sensor 11, and a controller 12.

The photoreceptor 1 is composed of, for example, a cylindrical aluminum tube having a surface coated with a photosensitive layer such as an OPC layer or amorphous silicon, and is used for exposing an electrostatic image. The charging unit 2 is formed of, for example, a corona charger, and uniformly charges the surface of the photoconductor 1. The exposure unit 3 includes, for example,
A laser light, an LED line head, and the like are provided.
An electrostatic image is written with ED light.

The toner replenishing unit 4 includes, for example, a toner hopper for storing toner, a paddle roller to which a mylar sheet for taking out a predetermined amount of toner is fixed, a sponge replenishing roller for gradually replenishing toner to the developing unit 5, and a paddle roller. A replenishment plate for averaging the removed toner mass and replenishing the replenishment roller, a motor for rotating and driving each roller,
A two-component developer carrier housed in the developing unit 5 is intermittently replenished with toner from a toner hopper containing the toner and controlled to a predetermined toner density under the control of the controller 12. (See FIGS. 2 and 3).

The developing unit 5 is connected to, for example, the toner replenishing unit 4 and has a storage box for storing a two-component developer composed of a toner and a carrier. Magnet roller (developing roller) with magnetized poles alternately, blade for frictionally charging toner and carrier, screw-type supply roller for transporting two-component developer to developing roller, motor for rotating each roller, motor drive circuit, etc. The two-component developer is raised on the developing roller to develop a toner image on the surface of the photoconductor 1 to which the electrostatic image has been exposed (see FIG. 3). For example, as a two-component developer, the carrier is composed of ferrite having a particle diameter of 50 to 60 microns, and the toner is composed of a carbon resin of 8 microns.

The conductive transfer section 6 is composed of an ion conductive transfer roller containing, for example, ammonium tetrachloride, and is disposed below the photosensitive member 1 to transfer a toner image developed on the photosensitive member 1 to a transfer material. Transfer to (paper) P. The ionic conductive transfer roller 6 is compared with the electronic conductive transfer roller.
There is little resistance non-uniformity, high resistance is easy to make, and the resistance value changes almost linearly with humidity change, so that accurate humidity can be measured. Thereby, the magnetic permeability of the two-component developer can be accurately corrected.

The voltage supply unit 7 includes a power supply for generating a constant DC voltage (bias voltage Vb) of, for example, several kV or less, and a voltage supply circuit, and has a polarity opposite to the charging polarity of the toner image developed on the photoreceptor 1. Is supplied to the conductive transfer section 6. When the bias voltage Vb is supplied to the conductive transfer unit 6, the toner image developed on the photoconductor 1 is transferred to the transfer material P. In addition, the voltage supply unit 7 can supply a reverse bias minute constant current It (for example, 2 μA) to the conductive transfer unit 6 when the humidity measuring unit 12 d measures the electric resistance value of the conductive transfer unit 6. It is configured.

The cleaning unit 8 is composed of, for example, a blade or an elastic roller, and removes residual toner from the surface of the photoconductor 1 after transfer. The charge removing section 9 is formed of, for example, an electromagnetic coil, and removes the charge on the cleaned photoconductor 1. The fixing unit 10 includes, for example, a heat roller and a pressure roller, and thermally fixes the toner image transferred to the transfer material P.

The magnetic permeability sensor 11 is composed of, for example, a ferrite and a coil, is disposed below the developing unit 5, and is housed in the developing unit 5 by detecting a change in inductance generated from a magnetic material of the carrier. The magnetic permeability of the component developer is detected.

The controller 12 comprises, for example, a CPU of a computer, a program memory for storing control programs and control data for controlling various parts of the apparatus, a buffer memory for storing control results and various measured data, and setting keys. Is done. The program memory also
It has a storage area that functions as a resistance-humidity conversion storage unit 12a and a permeability / humidity change characteristic storage unit 12b. The CPU of the controller 12 functions as a setting unit 12c, a humidity measurement unit 12d, a permeability correction unit 12e, and a toner supply control unit 12f by executing each control program.

The resistance-humidity conversion storage unit 12a stores a plurality of resistance-humidity conversion formulas for converting the electrical resistance value of the conductive transfer unit 6 into humidity. The permeability / humidity change characteristic storage unit 12b stores in advance the humidity change characteristics of the magnetic permeability for a predetermined toner concentration.

The setting unit 12c stores the resistance / humidity conversion type storage unit 1.
The resistance-humidity conversion formula corresponding to the humidity change characteristic of the electric resistance value of the conductive transfer portion 6 is read from 2d and set.

The humidity measuring section 12d is configured to measure the electric resistance value of the conductive transfer section 6, and to measure the humidity from the electric resistance value. Further, the humidity measuring unit 12d
A minute constant current of reverse bias is supplied from the voltage supply unit 7 to the conductive transfer unit 6 for a predetermined time before image formation, and the reverse bias voltage Vt applied to the conductive transfer unit 6 is measured, so that the electrical transfer of the conductive transfer unit 6 is performed. It is configured to measure a resistance value.
Further, the humidity measuring unit 12d is configured to convert the electric resistance value of the conductive transfer unit 6 into humidity based on the resistance / humidity conversion formula set by the setting unit 12e.

The magnetic permeability correction unit 12e is configured to correct the magnetic permeability that changes according to the humidity based on the humidity measured before the image formation by the humidity measurement unit 12a. The toner replenishment control unit 12f is configured to control the toner replenishment unit 4 based on the magnetic permeability corrected before image formation so that the toner concentration of the two-component developer always becomes a predetermined set value during image formation. ing.

FIG. 2 is a diagram showing the configuration of the toner supply section of this embodiment. As shown in FIG. 2, the toner replenishing section 4 includes a toner hopper 4a for storing toner, a paddle roller 4b to which a mylar sheet for taking out a predetermined amount of toner is fixed,
A replenishing roller 4c for sponge for gradually replenishing the toner to the developing unit 5 and a replenishing plate 4 for replenishing the replenishing roller 4c with an average of the toner mass taken out by the paddle roller 4b.
d, a motor (not shown) for rotating each of the rollers 4b and 4c, a motor drive circuit (not shown), and the like. The toner is supplied from the toner hopper 4a containing the toner to the developing unit 5 under the control of the toner replenishment control unit 12f. The stored two-component developer carrier is intermittently replenished with toner, and is controlled to a predetermined toner concentration. In this embodiment, the toner replenishment control unit 12f controls the toner concentration to be 7 ± 0.1% by weight.

FIG. 3 is a diagram showing the structure of a developing unit to which the toner supply unit of this embodiment is connected. As shown in FIG. 3, the developing unit 5 is connected to the toner replenishing unit 4, and has a storage box 5a for storing a two-component developer composed of a toner and a carrier. A developing roller (magnet roller) 5b having magnetized poles and S poles alternately; a blade 5c for frictionally charging the toner and the carrier; two screw-type supply rollers 5d and 5e for transporting the two-component developer to the developing roller 5b; A motor (not shown) that drives the rollers 5b, 5d, and 5e to rotate, a motor drive circuit (not shown), and the like are used. The two-component developer is raised on the developing roller 5b to expose the electrostatic image. A toner image is developed on the surface of the photoconductor 1.

Actually, the developing roller 5b is fixed, and the aluminum sleeve formed on the outer peripheral surface is rotating. Further, a magnetic permeability sensor 11 is arranged below the developing unit 5. Note that the configuration of the toner replenishing unit 4 is denoted by the same reference numeral as in FIG. 2, and description thereof is omitted here.

FIG. 4 is a diagram showing a change in the electric resistance value of the ionic conductive transfer roller of the present embodiment in humidity. FIG. 4A is a diagram showing an example of a measurement result of the resistance value of the ion-conductive transfer roller with respect to a change in humidity. Further, FIG.
It is the figure which expressed the measurement result of (a) by the linear linear graph.
In FIG. 4, for example, E + 08 indicates 10 to the eighth power.

As shown in FIG. 4B, the resistance value of the ion conductive transfer roller 6 (conductive transfer portion) decreases in proportion to the rise in humidity. For example, the resistance equation can be represented by R = -aH + b. Here, R is a resistance variable, H is a humidity variable, and a and b are positive constants set from measurement results.

The humidity can be calculated by measuring the resistance value of the ion-conductive transfer roller 6 and substituting it into a resistance formula showing the relationship between the change in humidity and the resistance value of the ion-conductive transfer roller. When a small constant current It is supplied from the supply voltage unit 7 to the ion conductive transfer roller 6,
Reverse bias voltage Vt applied to ion conductive transfer roller 6
Is measured, and the reverse bias voltage Vt is represented by a humidity formula showing the relationship between the change in humidity and the reverse bias voltage Vt of the ion conductive transfer roller 6.
The humidity can be calculated by substituting. For example, the reverse bias voltage equation is as follows: Vt = (− aH + b) · It
It can be represented by the following linear expression. Here, Vt indicates a reverse bias voltage variable, H indicates a humidity variable, and It indicates a minute constant current value.

FIG. 5 is a diagram showing a change in the humidity of the output value of the magnetic permeability sensor of this embodiment. FIG. 5A is a diagram illustrating an example of a change in the humidity of the output value of the magnetic permeability sensor with respect to the developer having a predetermined toner concentration. Here, since the toner density can be obtained in proportion to the magnetic permeability of the two-component developer, the output value of the magnetic permeability sensor = the toner density. FIG. 5B shows FIG.
It is the figure which represented the humidity change of the output value of the magnetic permeability sensor of (a) with the linear linear graph. In FIG. 5, here, w% indicates the weight percentage of the toner concentration = the output value of the magnetic permeability sensor.

As shown in FIG. 5B, the output value of the magnetic permeability sensor decreases as the humidity H increases. The output change equation can be expressed by a linear equation of O = -cH + d. Here, O is an output variable of the magnetic permeability sensor, H is a humidity variable, c,
d indicates a positive constant set from the measurement result.

Here, the output change equation for a toner concentration of 7% is represented by O = −0.0093H + 7.4792. The output change of the magnetic permeability sensor with respect to the humidity can be calculated by substituting the calculated humidity into an output change equation indicating a change in humidity of the output value of the magnetic permeability sensor. This output change equation can be used for correcting the output value of the magnetic permeability sensor for a predetermined toner concentration.

FIG. 6 is a flowchart showing a procedure for controlling the toner density according to this embodiment. 6, the CPU of the controller 12 controls the toner concentration to a predetermined set value (7%). Step S1: Before the paper P is transported, the ion conductive transfer roller 6 is idled for a predetermined time. Step S2: A constant current of 2 μA (microampere) is supplied from the voltage supply unit 7 to the ion conductive transfer roller 6. Step S3: The reverse bias voltage Vt applied to the ion conductive transfer roller 6 is measured and stored in the buffer memory.

Step S4: Reverse bias voltage Vt is 20
It is determined whether the voltage is less than 0 V (volt). Step S5: If the reverse bias voltage Vt is less than 200 V (volt), for example, Vt = (− aH + b) · It
The humidity is calculated by substituting the reverse bias voltage Vt into the equation of H = −0.223 Vt + 91, which is modified from the equation and the constant is determined.

Step S6: Reverse bias voltage Vt is 20
If it is 0 V (volt) or more, for example, Vt = (− aH
+ B) H is transformed from the formula of It and the constant is determined.
The humidity is calculated by substituting the reverse bias voltage Vt into the equation of −0.0115 Vt + 47. When supplying a reverse bias constant current of 2 μA (microamperes) to the ion conductive transfer roller 6 to obtain humidity from the reverse bias voltage Vt applied to the ion conductive transfer roller, the reverse bias voltage Vt is actually 200 V ( (B) depending on whether it is less than or equal to (volts), two equations having different primary equation constants a and b shown in FIG.

Step S7: Output change equation (O = -0.0) with respect to humidity of the magnetic permeability sensor at a toner concentration of 7%.
093H + 7.4792) to the correction coefficient α = 7 / (− 0.000) of the output value of the magnetic permeability sensor with respect to the toner concentration of 7%.
93H + 7.4792). Step S8: The output value T of the magnetic permeability sensor 11 is measured,
Store in the buffer memory. Step S9: The toner density D of the developing unit 5 is D = α · T
Thus, the toner replenishment control unit 12f controls the toner density D of the developing unit 5 to be 7%.

[0043]

According to the present invention, the humidity is measured from the electric resistance value of the conductive transfer portion for transferring the toner image developed on the photoreceptor to the paper, detected by the magnetic permeability sensor, and changed in accordance with the humidity. By correcting the magnetic permeability of the two-component developer, the humidity does not change even if the humidity changes in a method that does not use a humidity sensor.
The toner can be replenished so that the toner concentration of the component developer always becomes a predetermined set value. This makes it possible to always stabilize the printing quality even when the humidity changes.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration of a toner supply unit according to the present exemplary embodiment.

FIG. 3 is a diagram illustrating a configuration of a developing unit to which a toner supply unit according to the present embodiment is connected.

FIG. 4 is a diagram illustrating a humidity change of an electric resistance value of the ion conductive transfer roller of the present embodiment.

FIG. 5 is a diagram showing a change in humidity of an output value of the magnetic permeability sensor of the present embodiment.

FIG. 6 is a flowchart illustrating a procedure for controlling the toner density according to the embodiment.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photoconductor 2 charging unit 3 exposure unit 4 toner supply unit 5 developing unit 6 conductive transfer unit 7 voltage supply unit 8 cleaning unit 9 static elimination unit 10 fixing unit 11 permeability sensor 12 controller 12a resistance-humidity conversion storage unit 12b permeability Humidity change characteristic storage unit 12c Setting unit 12d Humidity measurement unit 12e Magnetic permeability correction unit 12f Toner supply control unit P Transfer material (paper)

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Hiroshi Ishii F-term in Sharp Corporation 22-22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka F-term (reference) 2H027 DA01 DA14 DD07 DE04 DE07 EA06 EC06 ED10 ED24 EE08 EF06 EF09 2H032 AA05 BA05 CA02 CA12 CA13 CA14 2H077 AA15 AA20 AB02 AB03 AB13 AB14 AB18 AB23 AC02 AC03 AD02 AD06 AE03 AE06 BA02 BA03 DA10 DA18 DA42 DA52 DB02 DB14 EA01 GA02

Claims (7)

[Claims] 1. A photoconductor for exposing an electrostatic image, a developing unit containing a two-component developer and developing a toner image on the surface of the photoconductor exposed to the electrostatic image, and supplying toner to the developing unit A toner replenishing unit, a conductive transfer unit that transfers a toner image developed on the surface of the photoconductor to a transfer material, and a voltage that supplies a predetermined bias voltage having a polarity opposite to the charging polarity of the toner image to the conductive transfer unit. A supply unit, a magnetic permeability sensor that detects the magnetic permeability of the two-component developer housed in the developing unit, and a humidity measuring unit that measures the electric resistance of the conductive transfer unit and measures humidity from the electric resistance. A magnetic permeability correction unit that corrects the magnetic permeability that changes in accordance with the humidity based on the measured humidity; and a magnetic permeability correction unit that corrects the toner concentration of the two-component developer so that the toner concentration always becomes a predetermined set value. And a toner replenishment control unit for controlling the toner replenishment unit. An image forming apparatus comprising: 2. The image forming apparatus according to claim 1, wherein said conductive transfer section is formed of an ion conductive transfer roller. 3. The conductive transfer unit according to claim 1, wherein the humidity measuring unit supplies a reverse bias minute constant current to the conductive transfer unit for a predetermined time before forming an image, and measures a reverse bias voltage applied to the conductive transfer unit. The image forming apparatus according to claim 1, wherein an electric resistance value of the unit is measured. 4. A resistance-humidity conversion storage unit in which a plurality of resistance-humidity conversion formulas for converting an electric resistance value of the conductive transfer unit into humidity are stored in advance, and an electric resistance of the conductive transfer unit from the resistance-humidity conversion storage unit is stored. A setting unit that sets a resistance-humidity conversion formula according to the humidity change characteristic of the value, wherein the humidity measurement unit converts the electrical resistance value of the conductive transfer unit into humidity based on the set resistance-humidity conversion formula. The image forming apparatus according to claim 1, wherein: 5. A magnetic permeability / humidity change characteristic storage unit in which a humidity change characteristic of magnetic permeability with respect to a predetermined toner concentration is stored in advance, wherein the humidity measuring unit refers to the magnetic permeability / humidity change characteristic storage unit, The image forming apparatus according to claim 4, wherein the magnetic permeability of the two-component developer detected by the magnetic permeability sensor is corrected based on the humidity measured by the measuring unit. 6. The magnetic permeability correction unit corrects the magnetic permeability based on the humidity measured before the image formation by the humidity measurement unit, and the toner supply control unit corrects the magnetic permeability during the image formation before the image formation. 2. The image forming apparatus according to claim 1, wherein the toner replenishing unit is controlled so that the toner concentration of the two-component developer always becomes a predetermined set value based on the magnetic permeability. 7. An image forming method for forming an image using the image forming apparatus according to claim 1.